Frequency converter and radio communications system employing the same

ABSTRACT

In a frequency converter, a phase-locked loop generates a local-oscillation signal having a low frequency, of a plurality of local-oscillation signals having different frequencies, based on an intermediate frequency beacon signal that results from mixing a predetermined beacon signal with the local-oscillation frequency signal. Even if the phase-locked loop is used to generate the low frequency local-oscillation signal only, a frequency offset and a phase noise taking place in remaining high frequency local-oscillation signals are compensated for or canceled out. The frequency converter thus results in a high frequency accuracy. This arrangement reduces the number of bulky, costly and power-consuming phase-locked oscillators, typically used in the quasi millimeter band or the millimeter band. A simplified, compact frequency converter is thus provided, reducing both installation and operating costs.

This application is a Continuation of nonprovisional application Ser.No. 09/350,997 filed Jul. 12, 1999 now U.S. Pat. No. 6,724,804. Priorityis claimed based on U.S. Ser. No. 09/350,997 filed Jul. 12, 1999, whichclaims the priority of Japanese application 10197173 filed on Jul. 13,1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to frequency converters and radiocommunications systems employing the frequency converter and, moreparticularly, to a frequency converter and a radio communication, inwhich a received radio frequency signal is converted into anintermediate frequency signal and an intermediate frequency signal to betransmitted is converted into a radio frequency signal.

2. Description of the Related Art

There is a growing demand for a high-speed network as datacommunications rapidly gain widespread use. High-speed communicationsprovided by wired networks are still very expensive for individualusers, and local radio communications networks providing a low-costservice are now actively studied and developed. Such a local radionetwork uses radio frequency bands, such as quasi millimeter waves (20GH–30 GHz) or millimeterwaves (30 GH–300 GHz), capable giving a highantenna gain with a small antenna. In the local radio network, a hubstation, for example, installed in a telephone exchange station,provides high-speed two-way data communications service or local TVphone service to a plurality of (user) subscriber stations within apredetermined coverage area.

When radio communications are performed using radio frequency signals inthe quasi millimeter band or the millimeter band, an intermediatefrequency signal of several tens to several hundreds of megahertz,rather than radio frequency signals, is subjected to a receiving processincluding an isolation decoding step, a transmitting process includingcoding and synthesis steps, and an amplification of signals. The cost ofa circuit arrangement required for the receiving and transmittingprocesses and the signal amplification is thus reduced. FIG. 16 shows afrequency converter, which converts a radio frequency signal into anintermediate frequency signal or an intermediate frequency signal into aradio frequency signal.

The frequency converter is installed in a subscriber station, forexample, and receives radio waves, transmitted from a hub station,through its receiving antenna 1001. Out of the radio waves received bythe receiving antenna 1001, radio frequency RF(RX) signals of interestfor reception in a plurality of frequency channels in a range of 22.6GHz–23.0 GHz are extracted by a bandpass filter 1002.

The radio frequency RF(RX) signal extracted through the bandpass filter1002 is amplified to an appropriate level by a low noise amplifier 1003,and is then mixed with a TX/RX local oscillation frequency signal LO1,for example, a 21 GHz signal, by a mixer 1004.

The local oscillation frequency signal LO1 is generated by aphase-locked oscillator 1100.

The phase-locked oscillator 1100 includes a phase-locked loop includinga counter circuit 1102, a frequency comparator 1103, a loop filter 1105,and a voltage-controlled oscillator 1106, and frequency multipliers 1107and 1109.

In the phase-locked oscillator 1100, a signal output by thevoltage-controlled oscillator 1106 is frequency-divided, for example, by175, by the counter circuit 1102. The frequency comparator 1103 comparesa signal output by the counter circuit 1102 to a reference signal, forexample, a 10 MHz reference signal supplied by a reference oscillator1204 employing a highly stable crystal oscillator. A voltage,corresponding to the difference between the two signals, is thenamplified by the loop filter 1105 in appropriate frequencycharacteristics. The voltage output from the loop filter 1105 is fedback to a control input of the voltage-controlled oscillator 1106.

A 1.75 GHz signal output by the voltage-controlled oscillator 1106 isfrequency-multiplied by four times by the frequency multiplier 1107,becoming a 7.0 GHz signal. The remaining signals contained in the outputof the frequency multiplier 1107 are filtered out by a bandpass filter1108.

The signal output by the bandpass filter 1108 is furtherfrequency-multiplied by three times by the frequency multiplier 1109,becoming a 21.0 GHz signal. The remaining signals contained in theoutput of the frequency multiplier 1109 are filtered out by a bandpassfilter 1110.

In this way, the phase-locked oscillator 1100 results in the signalhaving the local-oscillation frequency LO1 (21 GHz) at the samefrequency accuracy level as that provided by the highly stable referenceoscillator 1204.

The local-oscillation frequency signal LO1 is output by the bandpassfilter 1110, and is amplified by an amplifier 1112, and is then receivedby the RX mixer 1004.

An output of the mixer 1004 contains the frequency components of the sumof, and the difference between, the radio frequency RF(RX) signal andthe local-oscillation frequency LO1 signal. The difference between thetwo signals, i.e., a signal in an intermediate frequency band IF1(RX) of1.6 GHz–2.0 GHz, is extracted by the bandpass filter 1005, is amplifiedby an amplifier 1006, and is fed to an RX mixer 1007.

The RX mixer 1007 mixes the signal in the intermediate frequency bandIF1(RX) and a local-oscillation frequency LO2 signal, for example, a 1.1GHz signal supplied by a phase-locked oscillator 1200.

The local-oscillation frequency LO2 signal is generated by thephase-locked oscillator 1200.

The phase-locked oscillator 1200 includes a phase-locked loop includinga counter circuit 1202, a frequency comparator 1203, a loop filter 1205,and a voltage-controlled oscillator 1206, and the high-accuracyreference oscillator 1204 employing a crystal oscillator.

In the phase-locked oscillator 1200, a signal output by thevoltage-controlled oscillator 1206 is frequency divided, for example, by110, by the counter circuit 1102. The frequency comparator 1203 comparesa signal output by the counter circuit 1202 to a reference signal, forexample, a 10 MHz reference signal supplied by the reference oscillator1204. A voltage, corresponding to the difference between the twosignals, is then amplified by the loop filter 1205 in appropriatefrequency characteristics. The voltage output from the loop filter 1205is fed back to a control input of the voltage-controlled oscillator1206.

In this way, the phase-locked oscillator 1200 results in the signalhaving the local-oscillation frequency LO2 (1100 MHz) at the samefrequency accuracy level as that provided by the highly stable referenceoscillator 1204.

The output of the RX mixer 1007 contains frequency components of the sumof, and the difference between, the signal in the intermediate frequencyband IF1(RX) and the local-oscillation frequency LO2 signal. Thedifference between the two signals, i.e., a signal in an intermediatefrequency band IF2(RX) of 500 MHz–900 MHz, is extracted through thebandpass filter 1008.

The signal in the intermediate frequency band IF2(RX), picked up by thebandpass filter 1008, is amplified by an amplifier 1009, is fed to adiplexer 1010, and is then fed to a demodulator (not shown) via an IFcable.

The signal in the radio frequency band RF(RX) thus received is convertedinto a signal in an appropriate intermediate frequency band IF2(RX).

The signal in the intermediate frequency band IF2(TX), for example, 10MHz–60 MHz, supplied by a modulator (not shown), is received from thediplexer 1010 via the IF cable, is amplified by an amplifier 1012, andfed to a TX mixer 1013. The RX intermediate frequency IF2(RX) and the TXintermediate frequency IF2(TX) are assigned in separate frequencyranges.

The TX mixer 1013 mixes the signal in the intermediate frequency bandIF2(TX) with the signal having the local-oscillation frequency LO2output by the phase-locked oscillator 1200.

The output of the TX mixer 1013 contains the signals of the sum of, andthe difference between, the signal in the intermediate frequency bandIF2(TX) and the local-oscillation frequency LO2 signal. The signal ofthe sum of the two signals, i.e., a signal in an intermediate frequencyband IF1(TX) of 1.11 GHz–1.16 GHz, is extracted by a bandpass filter1014.

The signal in the intermediate frequency band IF1(TX), extracted by thebandpass filter 1014, is amplified by an amplifier 1015, and is then fedto a TX mixer 1016.

The TX mixer 1016 mixes the signal in the intermediate frequency bandIF1(TX) with the signal in the local-oscillation frequency LO1 output bythe phase-locked oscillator 1100.

The output of the TX mixer 1016 includes the signal components of thesum of, and the difference between, the signal in the intermediatefrequency band IF1(TX) and the signal having the local-oscillationfrequency LO1. The signal of the sum of the two signals, i.e., a signalin a radio frequency band RF(TX) of 22.11 GHz–22.16 GHz, is extracted bya bandpass filter 1017.

The signal in the radio frequency band RF(TX), extracted by the bandpassfilter 1017, is amplified to an appropriate level, and is fed to atransmitting antenna 1019. The corresponding radio wave is thentransmitted through the transmitting antenna 1019 to the hub station.

In this way, a signal in the appropriate intermediate frequency bandIF2(TX) is frequency converted into a signal in the radio frequency bandRF(RX) and is then transmitted.

The conventional frequency converter employs the phase-lockedoscillators 1100 and 1200 to respectively generate a plurality ofsignals in the local-oscillation frequencies LO1 and LO2, shared by thetransmitter part and the receiver part.

The phase-locked oscillators having a crystal oscillator working in thequasi millimeter band or the millimeter band are generally costly. Theyare complex and bulky, requiring a substantial maintenance cost, andconsume much power, requiring a substantial operating cost. As a result,if the frequency converter is built in a subscriber station in a localradio network (a radio communications system), the cost of eachindividual subscriber's subscriber station increases much.

When the local-oscillation frequency is generated using the phase-lockedoscillator, the frequency accuracy inevitably degrades in proportion tothe ratio N of the frequency of the output signal to the frequency ofthe reference signal in the crystal oscillator, and a phase noiseinevitably increases in proportion to the square of N. As thelocal-oscillation frequency becomes higher, the attained accuracy levelof the output frequency and the level of the phase noise are limitedmore.

For example, when the frequency accuracy of the 10 MHz referenceoscillator is +/−10 Hz (1E-6) and the phase noise is −120 dBc/Hz at anoffset of 1 kHz, N=21 GHz/10 MHz=2100 at a frequency of 21 GHz. Theattained frequency accuracy level is +/−21 kHz, and the phase noiselevel is −54 dBc/Hz at an offset of 1 kHz because 20 log 2100=66 dB.

If the local-oscillation frequency becomes higher, a frequencymultiplier may be required. The frequency multiplier can work as asource of unwanted radiated signals, degrading spurious characteristicsof the frequency converter.

These degraded characteristics lead to a drop in the utilization offrequencies when communications are performed using frequency divisionmultiplexing.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide afrequency converter which results in an improved accuracy level offrequency and implements a compact and low-cost design by employing aphase-locked oscillator for a relatively low local-oscillation frequencyonly. It is also an object of the present invention to provide a radiocommunications system which results in an improved utilization offrequencies and lightens the cost imposed on a subscriber byincorporating the frequency converter.

To achieve the above object, a frequency converter of the presentinvention, in one aspect, converts a signal in a first frequency band toa signal in a second frequency band by successively mixing the signal inthe first frequency band with a plurality of local-oscillation signalshaving different frequencies. The frequency converter includes aphase-locked loop, wherein the phase-locked loop generates alocal-oscillation signal having a low frequency, of the plurality oflocal-oscillation signals, based on an intermediate frequency beaconsignal, which is generated by mixing a predetermined radio frequencybeacon signal with the local-oscillation signal.

The phase-locked loop generates the local-oscillation signal having thelow frequency, of the plurality of local-oscillation signals, based onthe intermediate frequency beacon signal, which is generated by mixingthe predetermined radio frequency beacon signal with thelocal-oscillation frequency signal. Even if a frequency offset and aphase noise take place in a local-oscillation signal having a highfrequency, of the plurality of local-oscillation signals, the frequencyoffset and the phase noise are compensated for or canceled out when thesignal in the first frequency band is mixed with the local-oscillationsignal having the low frequency. Specifically, even if the phase-lockedloop is used in the frequency converter to generate the lowlocal-oscillation frequency signal only, the frequency offset and thephase noise taking place in the remaining high frequencylocal-oscillation signals are compensated for or canceled out. A highfrequency accuracy thus results. This arrangement reduces the number ofbulky, costly and power-consuming phase-locked oscillators, typicallyused in the quasi millimeter band or the millimeter band. A simplified,compact frequency converter is thus provided, reducing both installationand operating costs. The overall frequency accuracy of the frequencyconverter, employing the phase-locked loop, is improved. Furthermore,since a frequency multiplier is dispensed with, the spuriouscharacteristics of the frequency converter are improved.

In a preferred embodiment of the present invention, the phase-lockedloop may generate the local-oscillation signal having the low frequency,of the plurality of local-oscillation signals so that a differencebetween the frequency of the intermediate frequency beacon signal and apredetermined reference frequency becomes zero.

In a preferred embodiment of the present invention, the phase-lockedloop may generate the local-oscillation signal having the low frequency,of the plurality of local-oscillation signals so that the intermediatefrequency beacon signal is synchronized with a signal having apredetermined frequency.

The phase-locked loop generates the local-oscillation signal having thelow frequency, of the plurality of local-oscillation signals so that theintermediate frequency beacon signal is synchronized with thepredetermined frequency signal. Even when a frequency offset and a phasenoise take place in a signal local-oscillation having a high frequency,of the plurality of local-oscillation signals, the frequency offset andthe phase noise are compensated for or canceled out when the signal inthe first frequency band is mixed with the local-oscillation signalhaving the low frequency, of the plurality of local-oscillation signals.

In a preferred embodiment of the present invention, the phase-lockedloop may synchronize the local-oscillation signal having the lowfrequency, of the plurality of local-oscillation signals, with theintermediate frequency beacon signal.

The phase-locked loop generates the local-oscillation signal having thelow frequency, of the plurality of local-oscillation signals, insynchronization with the intermediate frequency beacon signal. Even whena frequency offset and a phase noise take place in a local-oscillationsignal having a high frequency, of the plurality of local-oscillationsignals, the frequency offset and the phase noise are compensated for orcanceled out when the signal in the first frequency band is mixed withthe local-oscillation signal having the low frequency, of the pluralityof local-oscillation signals.

In a preferred embodiment of the present invention, the signal in thefirst frequency band may be a radio frequency signal while the signal inthe second frequency band may be an intermediate frequency signal.

The construction of the receiver part of the frequency converter forconverting the radio frequency signal into the intermediate frequencysignal is simplified and is made compact. The installation cost andoperating cost of the converter are thus reduced.

In a preferred embodiment of the present invention, the signal in thefirst frequency band may be an intermediate frequency signal while thesignal in the second frequency band may be a radio frequency signal.

The construction of the transmitter part of the frequency converter forconverting the intermediate frequency signal into the radio frequencysignal is simplified and is made compact. The installation cost andoperating cost of the converter are thus reduced.

A radio communications system of the present invention, in anotheraspect, includes a hub station and at least one subscriber station radiolinked to the hub station. The subscriber station communicates with thehub station by converting a signal in a first frequency band to a signalin a second frequency band by successively mixing the signal in thefirst frequency band with a plurality of subscriber stationlocal-oscillation signals having different frequencies. The hub stationtransmits, to the subscriber station, a beacon signal having a radiofrequency that is generated by mixing a signal having a hub stationlocal-oscillation frequency with a beacon signal having a predeterminedhub station intermediate frequency. The subscriber station includes aphase-locked loop, wherein the phase-locked loop generates alocal-oscillation signal having a low frequency, of the plurality ofsubscriber station local-oscillation signals used in the subscriberstation, based on a subscriber station intermediate frequency beaconsignal, which is generated by mixing a predetermined radio frequencybeacon signal, transmitted from the hub station, with the subscriberstation local-oscillation signal.

The hub station transmits, to the subscriber station, the radiofrequency beacon signal into which the beacon signal having the hubstation intermediate frequency and the signal having the hub stationlocal-oscillation frequency are mixed. On the other hand, the subscriberstation communicates with the hub station by successively mixing thesignal in the first frequency band with the plurality of subscriberstation local-oscillation signals having the different frequencies. Thephase-locked loop generates a local-oscillation signal having a lowfrequency, of the plurality of local-oscillation signals, based on theintermediate frequency beacon signal, which is generated by mixing thepredetermined radio frequency beacon signal with the local-oscillationfrequency signal. Therefore, even when a frequency offset and a phasenoise take place in a local-oscillation signal having a high frequency,of a plurality of local-oscillation signals, the frequency offset andthe phase noise are compensated for or canceled out when the signal inthe first frequency band is mixed with the local-oscillation signalhaving the low frequency. Specifically, even if the phase-locked loop isused in the frequency converter to generate the low local-oscillationfrequency signal only, the frequency offset and the phase noise takingplace in the remaining high frequency local-oscillation signals arecompensated for or canceled out. A high frequency accuracy thus results.This arrangement reduces the number of bulky, costly and power-consumingphase-locked oscillators, typically used in the quasi millimeter band orthe millimeter band. A simplified, compact frequency converter is thusprovided, reducing both installation and operating costs and therebylightening the burden on the subscriber. The overall frequency accuracyof the frequency converter, employing the phase-locked loop, isimproved. Furthermore, since a frequency multiplier is dispensed with,the spurious characteristics of the frequency converter are improved.

In a preferred embodiment of the present invention, the phase-lockedloop may generate the subscriber station local-oscillation signal havingthe low frequency, of the plurality of subscriber stationlocal-oscillation signals so that a difference between the frequency ofthe subscriber station intermediate frequency beacon signal and apredetermined reference frequency becomes zero.

In a preferred embodiment of the present invention, the phase-lockedloop may generate the subscriber station local-oscillation signal havingthe low frequency, of the plurality of subscriber stationlocal-oscillation signals so that the subscriber station intermediatefrequency beacon signal is synchronized with a signal having apredetermined frequency.

In the subscriber station, the phase-locked loop generates thesubscriber station local-oscillation signal having the low frequency, ofthe plurality of subscriber station local-oscillation signals so thatthe subscriber station intermediate frequency beacon signal issynchronized with the predetermined frequency signal. Even when afrequency offset and a phase noise take place in a subscriber stationlocal-oscillation signal having a high frequency, of the plurality ofsubscriber station local-oscillation signals, and in the signal in thehub station local-oscillation frequency, the frequency offset and thephase noise are compensated for or canceled out when the signal in thefirst frequency band is mixed with the subscriber stationlocal-oscillation signal having the low frequency, of the plurality oflocal-oscillation frequency signals.

In a preferred embodiment of the present invention, the phase-lockedloop may synchronize the subscriber station local-oscillation signalhaving the low frequency, of the plurality of subscriber stationlocal-oscillation signals, with the subscriber station intermediatefrequency beacon signal.

The phase-locked loop generates the subscriber station local-oscillationsignal having the low frequency, of the plurality of subscriber stationlocal-oscillation signals, in synchronization with the subscriberstation intermediate frequency beacon signal. Even when a frequencyoffset and a phase noise take place in a subscriber stationlocal-oscillation signal having a high frequency, of the plurality ofsubscriber station local-oscillation signals, the frequency offset andthe phase noise are compensated for or canceled out when the signal inthe first frequency band is mixed with the subscriber stationlocal-oscillation signal having the low frequency, of the plurality ofsubscriber station local-oscillation signals.

In a preferred embodiment of the radio communications system of thepresent invention, the signal in the first frequency band may be a radiofrequency signal while the signal in the second frequency band may be anintermediate frequency signal.

The construction of the receiver part of the subscriber station of theradio communications system for converting the radio frequency signalinto the intermediate frequency signal is simplified and is madecompact. The installation cost and operating cost of the radiocommunications system are reduced.

In a preferred embodiment of the radio communications system of thepresent invention, the signal in the first frequency band may be anintermediate frequency signal while the signal in the second frequencyband may be a radio frequency signal.

The construction of the transmitter part of the subscriber station ofthe radio communications system for converting the intermediatefrequency signal into the radio frequency signal is simplified and ismade compact. The installation cost and operating cost of the radiocommunications system are reduced.

In a preferred embodiment of the present invention, a predeterminedfrequency range used by the hub station and the subscriber station isdivided into a plurality of frequency channels.

Since the radio communications system employs the phase-locked loop forthe low frequency local-oscillation signal only, the overall frequencyaccuracy of the system is improved and the utilization of frequencies isimproved.

A frequency converter of the present invention, another aspect, convertsa signal in a first frequency band to a signal in a second frequencyband by successively mixing the signal in the first frequency band witha plurality of local-oscillation signals having different frequencies,wherein an intermediate frequency signal, into which a spread spectrumreference signal having a predetermined frequency and alocal-oscillation frequency signal are mixed, is despread to result in areference signal, and based on the resulting reference signal, alocal-oscillation signal having a low frequency, of the plurality oflocal-oscillation signals having the different frequencies is generated.

In a preferred embodiment of the present invention, the frequency of thelocal-oscillation signal having the low frequency, of the plurality oflocal-oscillation signals, may be determined based on the level of thereference signal that is obtained by despreading the intermediatefrequency signal.

In the frequency converter, the frequency of the local-oscillationsignal having the low frequency, of the plurality of local-oscillationsignals, is set based on the level of the reference signal that isobtained by despreading the intermediate frequency signal. When thereference signal is correctly despread, the reference signal obtainedthrough the despreading process exceeds a predetermined threshold level.The intermediate frequency then is regarded as being coincident with thetransmission frequency when the reference signal is spread. Even when afrequency offset and a phase noise take place in a subscriber stationlocal-oscillation signal having a high frequency, of a plurality oflocal-oscillation frequency signals, the frequency offset and the phasenoise are compensated for or canceled out, and a high accuracy level offrequency is achieved. Since the reference signal is overlapped in thesame frequency band as that of data signal, through the spread spectrummodulation, with almost no effect incurred on the data signal, no bandoutside the frequency band for the data signal is consumed in a uselessfashion, and generally limited frequency bands are efficiently utilized.Since there is no need for crystal oscillators or the like, a simple andcompact design may be promoted further, and the installation andoperating costs of the converter are reduced.

In a preferred embodiment of the present invention, the signal in thefirst frequency band may be a radio frequency signal while the signal inthe second frequency band may be an intermediate frequency signal.

The construction of the receiver part of the frequency converter forconverting the radio frequency signal into the intermediate frequencysignal is simplified and is made compact. The installation cost andoperating cost of the converter are reduced.

In a preferred embodiment of the present invention, the signal in thefirst frequency band may be an intermediate frequency signal while thesignal in the second frequency band may be a radio frequency signal.

The construction of the transmitter part of the frequency converter forconverting the intermediate frequency signal into the radio frequencysignal is simplified and is made compact. The installation cost andoperating cost of the converter are reduced.

A radio communications system of the present invention, in yet anotheraspect, includes a hub station and at least one subscriber station radiolinked to the hub station, wherein the subscriber station communicateswith the hub station by converting a signal in a first frequency band toa signal in a second frequency band by successively mixing the signal inthe first frequency band with a plurality of subscriber stationlocal-oscillation signals having different frequencies. The hub stationtransmits, to the subscriber station, a spread spectrum signal that isgenerated by spreading a predetermined reference signal and then bymixing a hub station local-oscillation frequency signal with the spreadspectrum reference signal. A subscriber station local-oscillation signalhaving a low frequency, of the plurality of subscriber stationlocal-oscillation signals used in the subscriber station, is generatedfrom a reference signal that is obtained by despreading a subscriberstation intermediate frequency signal which results from mixing thespread spectrum signal transmitted from the hub station and thesubscriber station local-oscillation frequency signal.

In a preferred embodiment of the present invention, the frequency of thesubscriber station local-oscillation signal having the low frequency, ofthe plurality of subscriber station local-oscillation signals, may bedetermined based on the level of the reference signal that is obtainedby despreading the intermediate frequency signal.

In the radio communications system, the hub station transmits, to thesubscriber station, the spread spectrum signal that is obtained bysubjecting the predetermined reference signal to the spread spectrumprocess, and by mixing the spread spectrum reference signal with the hubstation local-oscillation frequency signal. To communicate with the hubstation, the subscriber station converts the signal in the firstfrequency band to the signal in the second frequency band, bysuccessively mixing the signal in the first frequency band with theplurality of subscriber station local-oscillation signals havingdifferent frequencies. In the frequency conversion process, thesubscriber station local-oscillation signal having the low frequency, ofthe plurality of subscriber station local-oscillation signals, is set infrequency so that the level of the reference signal that is obtained bydespreading the subscriber station intermediate frequency signal exceedsa predetermined threshold. When the reference signal is correctlydespread, the reference signal obtained through the despreading processexceeds the predetermined threshold level. The subscriber stationintermediate frequency then is regarded as being coincident with thetransmission frequency when the reference signal is spread. Even when afrequency offset and a phase noise take place in a subscriber stationlocal-oscillation signal having a high frequency, of a plurality oflocal-oscillation signals, the frequency offset and the phase noise arecompensated for or canceled out, and a high accuracy level of frequencyis achieved. Since the reference signal is overlapped in the samefrequency band as that of data signal, through the spread spectrummodulation, with almost no effect incurred on the data signal, no bandoutside the frequency band for the data signal is consumed in a uselessfashion, and generally limited frequency bands are efficiently utilized.Since there is no need for crystal oscillators or the like, a simple andcompact design may be promoted further, and the installation andoperating costs of the converter are reduced.

In a preferred embodiment of the present invention, the signal in thefirst frequency band may be a radio frequency signal while the signal inthe second frequency band may be an intermediate frequency signal.

The construction of the receiver part of the radio communications systemfor converting the radio frequency signal into the intermediatefrequency signal is simplified and is made compact. The installationcost and operating cost of the radio communications system are reduced.

In a preferred embodiment of the present invention, the signal in thefirst frequency band may be an intermediate frequency signal while thesignal in the second frequency band may be a radio frequency signal.

The construction of the transmitter part of the radio communicationssystem for converting the intermediate frequency signal into the radiofrequency signal is simplified and is made compact. The installationcost and operating cost of the radio communications system are reduced.

In a preferred embodiment of the present invention, a predeterminedfrequency range used by the hub station and the subscriber station isdivided into a plurality of frequency channels.

Since the radio communications system employs the phase-locked loop forthe low local-oscillation frequency signal only, the overall frequencyaccuracy of the system is improved and the utilization of frequencies isimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show an outline of a radio communications system and afrequency assignment for the system, according to one embodiment of thepresent invention;

FIG. 2 is a block diagram showing a frequency converter of oneembodiment of the present invention;

FIGS. 3A and 3B are charts showing the frequency conversion carried outby the frequency converter;

FIG. 4 is a block diagram of the frequency converter according to analternate embodiment of the present invention;

FIGS. 5A and 5B are charts showing the frequency conversion carried outby the frequency converter of the alternate embodiment;

FIGS. 6A through 6C are spectrum charts of signals in a spread spectrumprocess;

FIG. 7 is a block diagram roughly showing a transmitter performing thespread spectrum process;

FIG. 8 is a timing diagram showing the relationship between a PNsequence and transmission data in the spread spectrum process;

FIG. 9 is a block diagram roughly showing a receiver performing thespread spectrum process;

FIG. 10A shows a bandwidth of a data signal, over which a referencesignal, shown in FIG. 10B, is spread, and FIG. 10B shows thepredetermined reference signal;

FIG. 11 is a block diagram of a hub station in a radio communicationssystem according to another embodiment;

FIG. 12 is a frequency assignment chart for the hub station;

FIG. 13 is a block diagram of a local oscillator used in a frequencyconverter according to another embodiment;

FIG. 14 is a frequency chart illustrating the frequency conversion ofthe frequency converter;

FIGS. 15A through 15C show how the reference signal is overlapped on thedata signal; and

FIG. 16 is a block diagram of a conventional frequency converter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention are now discussed, referring tothe drawings. The following embodiments of the present invention areexemplary and are not intended to limit the scope of the presentinvention. FIGS. 1A and 1B show an outline of a radio communicationssystem and a frequency assignment for the system, according to oneembodiment of the present invention. FIG. 2 is a block diagram showing afrequency converter of one embodiment of the present invention. FIGS. 3Aand 3B are charts showing the frequency conversion carried out by thefrequency converter. FIG. 4 is a block diagram of the frequencyconverter according to an alternate embodiment of the present invention.FIGS. 5A and 5B are charts showing the frequency conversion carried outby the frequency converter of the alternate embodiment. FIGS. 6A through6C are spectrum charts of signals in a spread spectrum process. FIG. 7is a block diagram roughly showing a transmitter performing the spreadspectrum process. FIG. 8 is a timing diagram showing the relationshipbetween a PN sequence and transmission data in the spread spectrumprocess. FIG. 9 is a block diagram roughly showing a receiver performingthe spread spectrum process. FIG. 10A shows a bandwidth of a datasignal, over which a reference signal, shown in FIG. 10B, is spread.FIG. 10B shows the predetermined reference signal. FIG. 11 is a blockdiagram of a hub station in a radio communications system according toanother embodiment. FIG. 12 is a frequency assignment chart for the hubstation. FIG. 13 is a block diagram of a local oscillator used in afrequency converter according to another embodiment. FIG. 14 is afrequency chart illustrating the frequency conversion of the frequencyconverter. FIGS. 15A through 15C show how the reference signal isoverlapped on the data signal.

Referring to FIG. 1A, the radio communications system according to oneembodiment of the present invention includes a hub station 2000 and atleast one (user) subscriber station 1000, which are two-way radio-linkedin a hub configuration using frequency channels in the quasi millimeterband and the millimeter band.

The hub station 2000 includes, in its transmitter 2001, a synthesizer2002 for synthesizing intermediate frequency IF(Hub) transmissionmodulation wave group including a plurality of modulation waves that aremodulated through 256 QAM (Quadrature Amplitude Modulation), anamplifier 2003 for amplifying the intermediate frequency IF(Hub) outputfrom the synthesizer 2002, a mixer 2004 for generating a radio frequencyRF(Hub) signal by mixing the intermediate frequency IF(Hub) signaloutput from the amplifier 2003 with a hub station local-oscillationfrequency LO(Hub) signal, a bandpass filter 2005 for extracting theradio frequency RF(Hub) signal output from the mixer 2004, and atransmitter power amplifier 2006. The hub station 2000 transmits theradio frequency RF(Hub) signal to the subscriber station 1000 throughits transmitting antenna 2007. The radio frequency signal from thesubscriber station 1000 is received by a receiving antenna 2101, and isfed to a receiver 2100, but the detail about the reception operation isomitted here, and remains unchanged from that of a conventionalcommunications system.

The feature of the hub station 2000 in the above radio communicationssystem is that the transmitter 2001 includes a beacon signal oscillator2009 for oscillating a beacon signal having a predetermined intermediatefrequency IF(Hub). The beacon signal oscillated by the beacon signaloscillator 2009 is subjected to synthesis, together with thetransmission modulation waves, in the synthesizer 2002, and is mixedwith the hub station local-oscillation frequency LO(Hub) by the mixer2004, becoming a radio frequency RF(Hub) beacon signal. The radiofrequency RF(Hub) beacon signal is fed to the bandpass filter 2005 forfiltering, then to the transmitter power amplifier 2006 foramplification, and is transmitted to the subscriber station 1000 throughthe transmitting antenna 2007.

FIG. 1B is a frequency assignment chart of the transmission modulationwave group and the beacon signal. For example, seven frequency channels,corresponding to the transmission modulation wave group, are evenlyassigned in an intermediate frequency IF(Hub) range from 650 MHz to 1.0GHz. An intermediate frequency IF(Hub), not interfering with thefrequency channels, for example, 600 MHz, is assigned to the beaconsignal. In this example, the beacon signal is the low end frequency ofthe intermediate frequency range IF(Hub) of 600 MHz to 1 GHz, as atransmission band. Alternatively, the high end frequency or anintermediate frequency of the transmission range can be assigned to thebeacon signal. The signal in the range of 600 MHz to 1.0 GHz isamplified by the amplifier 2003, and is fed to the mixer 2004, where thesignal is mixed with a 22.0 GHz local-oscillation frequency LO(Hub)signal. The signal at each frequency channel is thus converted to aradio frequency RF(Hub) signal in a range of 22.6 GHz to 23.0 GHz. Theradio frequency RF(Hub) signal in the range of 22.6 GHz to 23.0 GHzoutput from the mixer 2004 is fed to the bandpass filter 2005, whichfilters out frequency components outside this range. The radio frequencyRF(Hub) is amplified by the transmitter power amplifier 2006 to asufficient level, and is then transmitted to the subscriber station 1000by the transmitting antenna 2007.

The subscriber station 1000 is provided with a frequency converter PFCshown in FIG. 2.

As shown, in the frequency converter PFC, RX mixers 1004 and 1007,respectively, mix the radio frequency RF(RX) signal with subscriberstation local-oscillation signals having different frequencies LO1 andLO2, and TX mixers 1013 and 1016, respectively, mix an intermediatefrequency IF(TX) signal with the subscriber station local-oscillationsignals having the different frequencies LO1 and LO2. The radiofrequency RF(RX) signal is thus converted into the intermediatefrequency IF(RX) signal, and the intermediate frequency IF(TX) signal isthus converted into the radio frequency RF(TX) signal. In thisoperation, the frequency converter PFC remains unchanged from theconventional frequency converter.

The frequency converter PFC is different from the conventional frequencyconverter in that the subscriber station local-oscillation signal havingonly the low frequency LO2, of the subscriber station local-oscillationfrequencies LO1 and LO2, is generated by the phase-locked oscillator1200 having a highly stable reference oscillator 1204. The phase-lockedoscillator 1200 generates the subscriber station local-oscillationfrequency LO2 signal so that the intermediate frequency signal IF2(RX)beacon signal is synchronized with a predetermined frequency signaloutput from the reference oscillator 1204. Here, the intermediatefrequency signal IF2(RX) beacon signal results from mixing the radiofrequency RF(RX) beacon signal with the signals having the subscriberstation local-oscillation frequencies LO1 and LO2.

Referring to FIG. 1A through FIG. 3B, the radio communications system,particularly, the frequency converter PFC, is discussed in detail. FIGS.3A and 3B are frequency charts illustrating the frequency conversion bythe frequency converter.

In the radio communications system, the radio frequency signal in therange of 22.6 GHz to 23.0 GHz shown in FIG. 1B, transmitted from the hubstation 2000, is received through the receiving antenna 1001 of thesubscriber station 1000.

Out of the radio waves received through the receiving antenna 1001 ofthe subscriber station 1000, the signal having the radio frequencyRF(RX) in the range of 22.6 GHz to 23.0 GHz is extracted by the bandpassfilter 1002. As shown in FIG. 3A, the 22.6 GHz signal, out of the radiofrequency RF(RX) signals, is the beacon signal, and serves as areference when other frequency channel signals are converted. The radiofrequency RF(RX) signals (containing the beacon signal), extracted bythe bandpass filter 1002, are amplified to an appropriate amplitude bythe low noise amplifier 1003, and are then mixed with the TX/RXlocal-oscillation frequency (subscriber station local-oscillationfrequency) LO1 signal, for example, a 21.0 GHz signal, by the RX mixer1004.

The local-oscillation frequency LO1 signal is generated by a localoscillator 1120 employing a dielectric oscillator or the like. The localoscillator 1120 employing a dielectric resonator gives a frequencyaccuracy of +/−1 MHz or so, and results in a high level phase noise inthe vicinity of the carrier, compared with the phase-locked oscillator.For this reason, the local-oscillation frequency LO1 signal contains afrequency drift LO1drift as large as 1 MHz (see FIG. 3A). The phasenoise is the phenomenon in which the frequency of the outputcontinuously varies in a random fashion at a relatively low frequency,and at a given moment, the phase noise is identical to a frequencyoffset.

The output of the RX mixer 1004 contains the frequency components of thesum of, and the difference between, the radio frequency RF(RX) signaland the local-oscillation frequency LO1 signal. The frequency componentof the difference, i.e., the signal, in the intermediate frequency rangeof IF1(RX) of 1.6 GHz to 2.0 GHz, is extracted by the bandpass filter1005, is amplified by the amplifier 1006, and is then fed to the RXmixer 1007.

The RX mixer 1007 mixes the signal in the intermediate frequency bandIF1(RX) and the local-oscillation frequency LO2 signal, for example, a1.1 GHz signal, supplied by the phase-locked oscillator 1200.

The output from the RX mixer 1007 contains the frequency components ofthe sum of, and the difference between, the intermediate frequencyIF(RX) signal and the local-oscillation frequency LO2 signal. Thefrequency component of the difference between both signals, i.e., thesignal in the intermediate frequency band IF2(RX) of 500 MHz to 900 MHz,is extracted by the bandpass filter 1008, is amplified by the amplifier1009, and is then fed to the diplexer 1010 and the phase-lockedoscillator 1200. The beacon signal contained in the intermediatefrequency IF2(RX) is 500 MHz. The signal in the intermediate frequencyband IF2(RX), fed to the diplexer 1010, is further fed to a demodulator(not shown) via an IF cable.

The intermediate frequency IF1 signal, fed to the RX mixer 1007,contains a frequency drift, as large as 1 MHz, derived from thelocal-oscillation frequency LO1 output from the local oscillator 1120,as shown in FIG. 3A.

In the frequency converter PFC and the radio communication system, ofthe present invention, the frequency drift is compensated for by thelocal-oscillation frequency LO2 signal output from the phase-lockedoscillator 1200.

The phase-locked oscillator 1200 includes the phase-locked loopincluding the counter circuit 1202, frequency comparator 1203, loopfilter 1205, and voltage-controlled oscillator 1206, and the referenceoscillator 1204 having a high accuracy crystal oscillator or the like,and a bandpass filter 1207.

In the phase-locked oscillator 1200, the bandpass filter 1207 extractsthe beacon signal, i.e., the low end frequency signal of 500 MHz fromthe intermediate frequency band IF2(RX) The beacon signal of theintermediate frequency signal IF2(RX), output from the bandpass filter1207, is frequency divided, for example, by 50, by the counter circuit1202, thereby becoming a signal near 10 MHz. The frequency comparator1203 compares the output from the counter circuit 1202 with the 10 MHzreference signal provided by the reference oscillator 1204. A voltagecorresponding to the difference between the two signals is amplified bythe loop filter 1205 in appropriate frequency characteristics. Thevoltage output from the loop filter 1205 is fed back to a control inputof the voltage-controlled oscillator 1206.

The local-oscillation frequency LO2 is thus output from the phase-lockedoscillator 1200 so that the frequency of the beacon signal becomes 500MHz at the same frequency accuracy level as that of the referenceoscillator 1204.

If the local-oscillation frequency LO1 becomes 1 MHz higher, thelocal-oscillation frequency output from the phase-locked oscillator 1200is set to be 1 MHz lower, and the beacon signal is thus stabilized at500 MHz at the frequency accuracy level of the reference oscillator1204.

The phase-locked oscillator 1200 thus corrects not only the beaconsignal but also the frequency drift in the intermediate frequency bandIF2(RX), which occurs when the signal in the radio frequency band RF(RX)is mixed with the local-oscillation frequency LO1 signal. As a result,the frequency conversion is carried out in an extremely stable manner,and the phase noise is minimized.

For example, when the frequency accuracy of the 10-MHz referenceoscillator is +/−10 Hz (1E-6) and the phase noise is −120 dBc/Hz at anoffset of 1 kHz, N=1 GHz/10 MHz=100 at an intermediate frequency of 1GHz. The attained frequency accuracy level is +/−1 kHz, and the phasenoise is as low as −80 dBc/Hz at an offset of 1 kHz because 20 log100=40 dB. This frequency accuracy level is 1000 times better than thatwhich is attained when a simple local oscillator 1120 (+/−1 MHz) havinga dielectric resonator is employed to generate the local-oscillationfrequency LO1 signal. Furthermore, this frequency accuracy level is 20times better than that when the phase-locked oscillator 1100 along withthe reference oscillator 1204 (+/−21 kHz) is employed to generate thelocal-oscillation frequency LO1 in the conventional art. With this levelof frequency accuracy, this embodiment works with a high-end frequencymodulation such as 256 QAM that features a narrow occupied frequencyband and efficient frequency utilization.

In this way, the signal in the radio frequency band RF(RX), transmittedfrom the hub station 2000 and received through the receiving antenna1001 of the subscriber station 1000, is converted into the intermediatefrequency IF2(RX) signal.

The local-oscillation frequency LO2, compensated for by the phase-lockedoscillator 1200, is used not only in the receiver but also in thetransmitter.

The signal in the intermediate frequency band IF2(TX) of 10 MHz to 60MHz, shown in FIG. 3B, and fed from a modulator (not shown) via the IFcable, is fed to the amplifier 1012 for amplification through thediplexer 1010 and is then fed to the TX mixer 1013. Different frequencybands are assigned to the RX intermediate frequency band IF2(RX) and theTX intermediate frequency band IF2(TX).

The TX mixer 1013 mixes the signal in the intermediate frequency bandIF2(TX) and the local-oscillation frequency LO2 signal output from thephase-locked oscillator 1200.

The output from the TX mixer 1013 contains the frequency components ofthe sum of, and the difference between, the signal in intermediatefrequency band IF2(TX) and the local-oscillation frequency LO2 signaloutput from the phase-locked oscillator 1200. The signal of the sum,i.e., the signal in the intermediate frequency band IF1(TX) of 1.11 GHzto 1.16 GHz, is extracted by the bandpass filter 1014 as shown in FIG.3B.

The signal in the intermediate frequency band IF1(TX), extracted by thebandpass filter 1014, is amplified by the amplifier 1015, and is fed tothe TX mixer 1016.

The TX mixer 1016 mixes the signal in the intermediate frequency bandIF1(TX) and a 21.0 GHz local-oscillation frequency signal LO1 outputfrom the local oscillator 1120.

The output from the TX mixer 1016 contains the frequency components ofsum of, and the difference between, the signal in the intermediatefrequency band IF1(TX) and the local-oscillation frequency LO1 signal.The signal of the sum, i.e., the signal in the radio frequency bandRF(TX) of 22.11 GHz to 22.16 GHz is extracted by the bandpass filter1017 as shown in FIG. 3B.

Since the local-oscillation frequency LO1 signal is generated by thelocal oscillator 1120, employing a dielectric resonator or the like, inthe same manner as in the receiver, the 21.0 GHz local-oscillationfrequency LO1 signal suffers from a frequency drift of 1 MHz or so. Thelocal-oscillation frequency LO2, already mixed, varies in accordancewith the frequency drift, and the frequency drift is thus compensatedfor when the local-oscillation frequency LO1 signal is mixed with theintermediate frequency IF1(TX) signal.

The radio frequency component RF(TX) signal, extracted by the bandpassfilter 1017, is amplified to an appropriate level by a power amplifier1018 and is then fed to the transmitting antenna 1019. From thetransmitting antenna 1019, the radio frequency RF(TX) signal istransmitted.

In this way, the signal in the intermediate frequency band IF2(TX) isconverted into the signal in the radio frequency band RF(TX), which isthen transmitted to the hub station 2000. The hub station 2000 receivesit through the receiving antenna 2101 of the receiver 2100.

If the output LO(Hub) of a local oscillator 2008 is used as thelocal-oscillation frequency of the frequency converter in the receiver2100, a frequency drift taking place in the local oscillator 2008 isalso canceled out on the same principle.

In the frequency converter and the radio communications system employingthe frequency converter, the low frequency local-oscillation signal ofthe plurality of local-oscillation signals having different frequenciesis generated by the phase-locked loop based on the intermediatefrequency beacon signal resulting from mixing the predetermined radiofrequency beacon signal with the local-oscillation frequency signal.Even if a frequency drift takes place in the high frequencylocal-oscillation signal of the plurality of subscriber stationlocal-oscillation signals, the frequency drift and phase noise arecompensated for or canceled out when the signal in the first frequencyband is mixed with the low frequency local-oscillation signal of theplurality of local-oscillation signals. Specifically, even if the lowlocal-oscillation frequency signal only is generated using thephase-locked loop in the frequency converter, the frequency offset andthe phase noise taking place in the remaining high local-oscillationfrequency signals are compensated for or canceled out. A high frequencyaccuracy thus results. This arrangement reduces the number of bulky,costly and power consuming phase-locked oscillators, typically used inthe quasi millimeter band or the millimeter band. A simplified, compactfrequency converter is thus provided, reducing both installation andoperating costs. The overall frequency accuracy of the frequencyconverter, employing the phase-locked loop, is improved. Furthermore,since a frequency multiplier is dispensed with, the spuriouscharacteristics of the frequency converter are improved.

Alternate Embodiment

In the above embodiment, the two local-oscillation frequencies LO1 andLO2 are used in the frequency conversion. The present invention is notlimited to the two local-oscillation frequencies. The present inventioncan be applied to a frequency converter, which employs three or morelocal-oscillation frequencies.

FIG. 4 shows a frequency converter PFC' and a radio communicationssystem employing the frequency converter PFC' in an alternate embodimentof the present invention.

As shown, RX mixers 1004 and 1007, respectively, mix the radio frequencyRF(RX) signal with the signals having the local-oscillation frequenciesLO1 and LO2, and TX mixers 1013 and 1016, respectively, mix anintermediate frequency IF4(TX) signal with the signals having thelocal-oscillation frequencies LO2 and LO1. The radio frequency RF(RX)signal is converted into the intermediate frequency IF4(RX) signal, andthe intermediate frequency IF4(TX) signal is converted into the radiofrequency RF(TX) signal. This method remains unchanged from theconventional art.

The frequency converter PFC' is different from the conventional art inthat a phase-locked loop synchronizes the low frequency LO2 signal ofthe signals having the two local-oscillation frequencies LO1 and LO2,with a beacon signal contained in the signal in the intermediatefrequency band IF1(RX) resulting from mixing the radio frequency RF(RX)signal and the local-oscillation frequency LO signal.

The frequency converter PFC' is now discussed, referring to FIG. 4 andFIGS. 5A and 5B. FIGS. 5A and 5B are frequency charts showing thefrequency conversion performed by the frequency converter PFC'.

In the frequency converter PFC', a signal in a radio frequency band of27.50 GHz to 28.35 GHz shown in FIG. 5A, transmitted from the hubstation 2000, is received by the subscriber station 1000 through itsreceiving antenna 1001.

The radio wave received by the receiving antenna 1001 of the subscriberstation 1000 is picked up by a diplexer 1020, and is fed to a bandpassfilter 1002. The bandpass filter 1002 extracts a signal in a radiofrequency band RF(RX) of 27.50 GHz to 28.35 GHz, which is of interestfor reception among received signals. A 27.50 GHz signal of the radiofrequency RF(RX) signals is a beacon signal, and serves as a referencewhen other frequency channel signals are converted. The radio frequencyRF(RX) signals (containing the beacon signal), extracted by the bandpassfilter 1002, are amplified to an appropriate amplitude by the low-noiseamplifier 1003, and are then mixed with the TX/RX local-oscillationfrequency LO1 signal, for example, a 26.55 GHz signal, by the RX mixer1004.

The local-oscillation frequency LO1 signal is generated by the localoscillator 1120 employing a dielectric oscillator or the like. The localoscillator 1120 employing a dielectric resonator gives a frequencyaccuracy of +/−1 MHz or so, and results in a high level phase noise inthe vicinity of the carrier, compared with the phase-locked oscillator.For this reason, the local-oscillation frequency LO1 signal contains afrequency drift LO1drift as large as 1 MHz (see FIG. 5A).

The output of the RX mixer 1004 contains the frequency components of thesum of, and the difference between, the radio frequency RF(RX) signaland the local-oscillation frequency LO1 signal. The frequency componentof the difference, i.e., the signal, in the intermediate frequency bandIF1(RX) of 950 MHz to 1800 MHz, is extracted by the bandpass filter1005, is then amplified by the amplifier 1006, and is then fed to the RXmixer 1007 and the bandpass filter 1207.

The RX mixer 1007 mixes the signal in the intermediate frequency bandIF1(RX) and a signal Synthe (RX) in a frequency range of 2950 MHz to3800 MHz, supplied by a frequency synthesizer 1301.

The output of the RX mixer 1007 contains the frequency components of thesum of, and the difference between, a signal in the intermediatefrequency band IF(RX) and the Synthe(RX) signal. The frequency componentof the difference, i.e., a 2000 MHz intermediate frequency IF2(RX)signal, is extracted by the bandpass filter 1008 according to itspassband characteristics. The signal, corresponding to a single channel,is thus selected. The output of the bandpass filter 1008 is fed to an RXmixer 1302 after being amplified by the amplifier 1009.

The RX mixer 1302 mixes the output of the amplifier 1009, i.e., the 2000MHz intermediate frequency IF2(RX) signal and a 3840-MHzlocal-oscillation frequency LO3(RX) signal that is higher in frequencythan the Synthe(RX) signal.

The output of the RX mixer 1302 contains the frequency components of thesum of, and the difference between the signal in the intermediatefrequency band IF2(RX) and the local-oscillation frequency LO3(RX)signal. The frequency component of the difference, i.e., a 1840-MHzintermediate frequency IF3(RX) signal, is extracted by a bandpass filter1304, is amplified by a bandpass filter 1305, and is then fed to an RXmixer 1306.

The RX mixer 1306 mixes the signal in the intermediate frequency bandIF3(RX) and a local-oscillation frequency LO2 signal having a frequencyof 1000 MHz or so. The local-oscillation frequency LO2 signal issupplied, through an amplifier 1208, by a voltage-controlled oscillator1206 included in the phase-locked oscillator 1200.

The frequency converter PFC' cancels out a frequency drift of about 1MHz in the signal in the intermediate frequency band IF3(RX), attributedto the local-oscillation frequency LO1, by mixing the local-oscillationfrequency LO2 signal with the signal in the intermediate frequency bandIF3(RX).

The frequency drift is canceled when the phase-locked oscillator 1200synchronizes the local-oscillation frequency LO2 signal to the beaconsignal contained in the signal in the intermediate frequency bandIF1(RX).

The beacon signal contained in the signal in the intermediate frequencyband IF1(RX) is extracted by the bandpass filter 1207 in thephase-locked oscillator 1200. The frequency of the beacon signalcontained in the signal in the intermediate frequency band IF1(RX) is ahigh end frequency of the intermediate frequency band IF1(RX), namely,1800 MHz. A frequency comparator 1203 compares the 1800 MHz beaconsignal, extracted by the bandpass filter 1207, to the local-oscillationfrequency LO2 signal output from the voltage-controlled oscillator 1206.A voltage corresponding to the difference between the two signals isamplified by a loop filter 1205 according to appropriate frequencycharacteristics. The voltage output from the loop filter 1205 is fedback to a control input of the voltage-controlled oscillator 1206.

In this way, the local-oscillation frequency LO2 varies, tacking thefrequency drift taking place in the beacon signal. If thelocal-oscillation frequency LO1 signal changes, becoming high infrequency by 1 MHz, the local-oscillation frequency LO2 output from thephase-locked oscillator 1200 drops by 1 MHz, thereby canceling thefrequency drift.

A signal, for example, a 40 MHz channel signal in an intermediatefrequency band IF4(RX), output by the RX mixer 1007 and extracted by abandpass filter 1307, is free from the effect of the frequency drift.The signal in the intermediate frequency band IF4(RX) is sent to ademodulator 1038 for demodulation.

A modulated signal in the intermediate frequency band IF4(TX), outputfrom a modulator 1310, is converted into a modulated signal in a radiofrequency band RF(TX) of 31.0 GHz to 31.3 GHz with a frequency driftsimilarly canceled out, as shown in FIG. 5B. The modulated signal is fedto the diplexer 1020, and is then transmitted through the antenna 1001.

In the frequency converter PFC' and the radio communications systememploying the frequency converter PFC', the subscriber stationlocal-oscillation signal having the low frequency, of thelocal-oscillation signals having different frequencies is generated insynchronization with the intermediate frequency beacon signal. Even if afrequency drift takes place in the high frequency local-oscillationsignal of the plurality of subscriber station local-oscillation signals,the frequency drift and phase noise are compensated for or canceled outwhen the signal in the first frequency band is mixed with the lowfrequency local-oscillation signal, of the plurality oflocal-oscillation signals.

Other Alternative Embodiments

In each of the above embodiments, the beacon signal and the transmissionmodulation wave group need to be set in different frequency bands tofacilitate separating the beacon signal from the transmission modulationwave group. The frequency band available for the transmission modulationwave group is narrowed by a portion of the frequency band occupied bythe beacon signal. In view of the utilization of frequencies only, theconventional art has an advantage over the above embodiments.

The inventors of this invention have developed a frequency converter anda radio communications system employing the frequency converter, whichgive the advantages as good as those of the preceding embodiments interms of bandwidth without occupying an extra frequency band other thanthe frequency band originally assigned to the transmission modulationwave group.

The spread spectrum modulation is a useful means that enables a signalto be contained in the same frequency band used for the transmissionmodulation wave group. The concept of the spread spectrum modulation isbriefly discussed here.

In the spread spectrum modulation, a frequency spectrum of a signal tobe transmitted is spread over along a frequency axis using anothersignal (a code) having a frequency spectrum substantially wider thanthat of the signal to be transmitted. The signal is thus transmittedusing a frequency bandwidth much wider than the frequency bandwidth itoriginally needs. FIGS. 6A, 6B and 6C are spectrum charts of a signal inthe spread spectrum modulation. A signal modulated with information,shown in FIG. 6A, is spread using a predetermined code. FIG. 6B shows aresulting spread spectrum signal having a wider frequency spectrum. Thisspread spectrum signal is transmitted. The spread spectrum signal, shownin FIG. 6B, has an extremely low level of power spectrum density andexerts no effect on other signals. In a receiver, the received signalshown in FIG. 6B is despread using a predetermined code (the same codeused in the spread spectrum modulation), and is thus demodulated (asshown in FIG. 6C). The spread spectrum signal (FIG. 6B) only is restoredto its original signal, shown in FIG. 6A, and other signals do notaffect the restored signal.

FIG. 7 and FIG. 9 are simplified block diagrams of a transmitter and areceiver, employing the spread spectrum technique. The transmitter andthe receiver shown here are not the examples in which the presentinvention is implemented, and are cited for the discussion purpose ofthe concept of the spread spectrum technique.

In the transmitter shown in FIG. 7, transmission data (shown in FIG. 6A,for example), already subjected to a primary modulation (phase-shiftkeying modulation in FIG. 7), is multiplied by a PN (pseudorandom noise)sequence (corresponding to the above-cited code). The transmission datais thus spread over a wider spectrum and is then transmitted. The PNsequence is a sequence of rectangular waves randomly taking +1 or −1 asshown in FIG. 8. The data rate (Tc in FIG. 8) of the randomly changingrectangular wave PN sequence is set to be substantially faster than thedata rate (T in FIG. 8) of the primary modulated signal (i.e., T>>Tc).Here, T/Tc is called a spreading ratio. The spreading ratio variesdepending on applications, but generally falls within a range of 10 to10000.

In the receiver shown in FIG. 9, the signal (i.e., the signal shown inFIG. 6B), received from the transmitter, is fed to a bandpass filterwhich removes unwanted frequency components contained in the signal. Thesignal is then multiplied by the PN sequence for despreadinq. The signalis thus demodulated back to the original primary modulation signal(i.e., the signal shown in FIG. 6C). Only when the PN sequences arealigned and timings (i.e., phases of the received signal and the PNsequence) coincide, the signal is demodulated. The demodulated signalpasses a narrow bandpass filter matching the band of the primarymodulation signal, and is then fed to an ordinary demodulator circuit.

From the above discussion, the spread spectrum technique has thefollowing advantage. Since the original signal is spread over afrequency band that is a spreading ratio times the original frequencybandwidth, the power spectrum density is the inverse of the spreadingratio. The spread spectrum signal exerts almost no effect on othersignals. The signal is then despread using the same code that is usedfor spreading, thus demodulated back to the original signal; and othersignals contained do not affect the demodulation. In other words, asignal contained in a narrow band is spread in the course of thedespreading process of a signal of interest, thereby having a low powerdensity. The signal, which is spread like thermal noise, has nocorrelation with the spreading code, and thus remains unchanged in thedespreading process.

The use of the spread spectrum having the above advantage permits areference signal to be included in the same frequency band as that ofdata signal. FIG. 10A shows the spectrum of a transmission signal at agiven channel. The data signal has a bandwidth W determined by its datarate. The data signal is typically scrambled so that it may spread overthe bandwidth rather than being biased. A transmission signal isproduced by overlapping, on the data signal, a reference signal which isspread using a spreading code different from the code that was used toscramble the data signal. When the transmission signal is demodulatedusing the same spreading code that was used for the spreading process,the reference signal is restored back to its original signal level asshown in FIG. 10B. The data signal remains unchanged in signal level,because the data signal is unrelated to the spreading code. If anappropriate spreading ratio is used, the signal level of the of thereference signal occupying a portion of the transmission signal issufficiently small to that of the data signal. Therefore, the S/N ratioof the data signal is not deteriorated in the reception of the datasignal. There is no problem with the reception of the reference signal,because the reference signal is sufficiently large relative to the datasignal subsequent to the despreading process. Specifically, when thebandwidth of the data signal is 6 MHz (corresponding to the bandwidth ofa television signal channel) and the bandwidth of the reference signalis 6 kHz, the reference signal is spread over a bandwidth of 6 MHz at aspreading ratio of 1000. If the overall power of the reference signal isnow one-hundredth the power of the data signal, a resulting S/N ratio is20 dB, because the reference signal is spread over the same bandwidth asthat of the data signal. When the reference signal is demodulated, thepower spectrum density of the reference signal is increased by 1000times, but the power spectrum density of the data signal remainsunchanged. If a filter having a bandwidth corresponding to that of thereference signal is used, a resulting S/N ratio is 1000×0.01:1, i.e., 10dB. Both S/N ratios work in the demodulation process. Since thereference signal at a low energy level is acceptable, the powerconsumption for transmitting the reference signal only is also small.

The signal, subsequent to the despreading process, is a narrow bandsignal corresponding to the primary modulation signal. The frequency ofthe received signal has to be at a frequency accuracy level matching theprimary modulation signal. To demodulate the reference signal by thedespreading process, not only accomplishing a phase matching between thePN sequences (i.e., a time synchronization) but also accomplishing afrequency matching (i.e., a frequency synchronization) is required. Inother words, if the reference signal is correctly demodulated byshifting the phase of the PN sequence, the frequency of the receivedsignal is accurately aligned with the signal in the receiver.

One example of the radio communications system utilizing the abovefeature is now discussed, referring to FIG. 11 through FIG. 13. FIG. 11shows the construction of a hub station. FIG. 13 shows an oscillator1500, corresponding to the phase-locked oscillator 1200 shown in FIG. 2.In this embodiment, the frequency converter used in the subscriberstation is almost the same as the frequency converter shown in FIG. 2,except for the oscillator 1500 that replaces the phase-locked oscillator1200. The process for the downlink from the hub station to thesubscriber station is discussed, and the discussion of the uplinkprocess is omitted here.

Referring to FIG. 11, a transmitter 2001′ of the hub station 2000′ inthis embodiment is different from the transmitter 2001 shown in FIG. 1as follows: a spread spectrum signal, having the same frequency band asthe transmission modulation wave group, is produced by multiplying apredetermined reference signal (having a predetermined bandwidthcentered on 700 MHz as represented by a dotted line in FIG. 12) by a PNsequence and is overlapped on the transmission modulation wave group.The construction of the transmitter 2001′ subsequent to this process isalmost identical to that of the transmitter 2001 shown in FIG. 1. Thetransmission modulation wave group with the spread spectrum referencesignal overlapped thereon has a frequency band of 500 to 900 MHz, asshown in FIG. 12, and is mixed with a 22.1 GHz local-oscillationfrequency LO(Hub) signal, thereby resulting in a radio frequency RF(Hub)signal in a frequency band of 22.6 GHz to 23.0 GHz.

In this radio communications system, the radio frequency signal in thefrequency band of 22.6 to 23.0 GHz shown in FIG. 12, transmitted fromthe hub station 2000′, is received by the receiving antenna 1001 of thesubscriber station (see FIG. 2). The construction of the frequencyconverter of the subscriber station in this embodiment is almostidentical to the frequency converter shown in FIG. 2, except for thelocal oscillator 1500 (shown in FIG. 13) that replaces the phase-lockedoscillator 1200. The process before the local oscillator 1500 is nowbriefly discussed referring to FIG. 2.

Out of the radio waves received through the receiving antenna 1001, thesignal having the radio frequency band RF(RX) of 22.6 GHz to 23.0 GHz isextracted by the bandpass filter 1002. The radio frequency RF(RX)signals, extracted by the bandpass filter 1002, are amplified to anappropriate amplitude by the low-noise amplifier 1003, and are thenmixed with the TX/RX local-oscillation frequency (subscriber stationlocal-oscillation frequency) LO1 signal, for example, a 21.0 GHz signal,by the RX mixer 1004.

The local-oscillation frequency LO1 signal is generated by a localoscillator 1120 employing a dielectric oscillator or the like. The localoscillator 1120 employing a dielectric resonator gives a frequencyaccuracy of +/−1 MHz or so, and results in a high level phase noise inthe vicinity of the carrier, compared with the phase-locked oscillator.For this reason, the local-oscillation frequency LO1 signal contains afrequency drift LO1drift as large as 1 MHz (see FIG. 14).

The output of the RX mixer 1004 contains the frequency components of thesum of, and the difference between, the radio frequency RF(RX) signaland the local-oscillation frequency LO1 signal. The frequency componentof the difference, i.e., the signal, in the intermediate frequency bandIF1(RX) of 1.6 GHz to 2.0 GHz, is extracted by the bandpass filter 1005,is then amplified by the amplifier 1006, and is then fed to the RX mixer1007.

The RX mixer 1007 mixes the signal in the intermediate frequency bandIF1(RX) and the local-oscillation frequency LO2 signal, for example, a1.1 GHz signal, supplied by the phase-locked oscillator 1500 (see FIG.13).

The output from the RX mixer 1007 contains the frequency components ofthe sum of, and the difference between, the intermediate frequencyIF(RX) signal and the local-oscillation frequency LO2 signal. Thefrequency component of the difference between both signals, i.e., theintermediate frequency IF2(RX) signal in the range of 500 MHz to 900MHz, is extracted by the bandpass filter 1008, is amplified by theamplifier 1009, and is then fed to the diplexer 1010 and thephase-locked oscillator 1500.

The intermediate frequency IF1 signal, fed to the RX mixer 1007,contains a frequency drift, as large as 1 MHz or so, derived from thelocal-oscillation frequency LO1 output from the local oscillator 1120,as shown in FIG. 13. In the frequency converter PFC and the radiocommunication system, of this embodiment, the frequency drift iscompensated for by the local-oscillation frequency LO2 signal outputfrom the phase-locked oscillator 1500.

The phase-locked oscillator 1500 (see FIG. 13) performs the despreadingdemodulation process by multiplying the input signal in the intermediatefrequency band IF2(RX) by the PN sequence (identical to the PN sequenceused for spreading the reference signal in the hub station 2000′). Thesignal resulting from the despreading demodulation process is fed to asignal level detector 1502 through a bandpass filter 1501 (having abandwidth BIF). The signal level detector 1502 detects the level of thesignal. A comparator 1503 compares the signal level detected by thesignal level detector 1502 to a predetermined threshold. The thresholdis set to be slightly lower than the level of a correctly demodulatedreference signal. If the comparator 1503 determines that the signallevel detected by the signal level detector 1502 is higher than thethreshold, both phase and frequency are correctly synchronized.

When the comparator 1503 determines that the signal level detected bythe signal level detector 1502 is higher than the threshold, a searchcontroller 1504 performs a time synchronization process. Specifically, aphase shift controller 1505 determines the output of the comparator 1503while making the output of the PN sequence generator 1506 lead (or lag)in phase. This determination process is iterated by a countcorresponding to the longest PN sequence period. When no synchronizationis achieved even after searching one period of the PN sequence, thefrequency must suffer from offset (specifically, a frequency drift dueto the local-oscillation frequency LO1 output from the local oscillator1120 occurs in the intermediate frequency band IF1).

A frequency controller 1507 controls a voltage-controlled oscillator1508 so that the received frequency is lower than the lowest frequencybased on the assumption of the maximum frequency offset (to performsignal search from low to high frequency). The local-oscillationfrequency LO2 is thus adjusted. A newly obtained signal in theintermediate frequency band IF2(RX) is then subjected to the timesynchronization process. When the signal level is not above thethreshold in the comparator 1503, the oscillation frequency of thevoltage-controlled oscillator 1508 is increased by BIF/2 and the aboveprocess is repeated.

The local oscillator 1500 repeats the above process. When the signallevel exceeds the threshold in the comparator 1503, the reference signalcontained in the signal in the intermediate frequency band IF2(RX) iscorrectly demodulated, and the intermediate frequency band IF2(RX) iscorrectly aligned with the transmission frequency band (i.e., 500–900MHz). In other words, a TX/RX frequency relationship is correctlyestablished.

In the frequency converter and the radio communications system of thisembodiment, the frequency drift taking place in the highlocal-oscillation frequency signal is canceled out without utilizing afrequency band outside the frequency band assigned to the transmissionmodulation wave group. A high frequency accuracy level is thus achieved.Since a crystal oscillator is not necessary, the construction of thefrequency converter is simplified and is made compact. The installationcost and operating cost of the converter are reduced.

The following methods are contemplated to overlap the spread spectrumreference signal onto the data signal (transmission modulation wavegroup) in the hub station 2000′ in the radio communications system ofthis embodiment: i) the reference signal is overlapped onto each datasignal channel (see FIG. 15A), ii) the reference signal is overlappedonto at least one data signal channel (see FIG. 15B), and iii) thereference signal is overlapped, straddling all data signal channels (seeFIG. 15C). The above embodiment has been discussed in connection withthe method iii) in which the reference signal straddles over all datasignal channels as shown in FIG. 15C. Other method or a combination ofthese methods can be employed.

Methods i) and ii), respectively, shown in FIGS. 15A and 15B, foroverlapping the spread spectrum reference signal onto individual datachannels are executed when the data signal is modulated. Method iii)shown in FIG. 15C for straddling the spread spectrum reference signalonto all data channels is executed when the frequency conversion isperformed after all data channel modulation waves are collected. Thedemodulation of the reference signal needs to be executed in methods ii)and iii), respectively shown in FIG. 15B and FIG. 15C, when all channelsignal are still present (before any particular channel is extracted).In case of method i), shown in FIG. 15A, the demodulation can beexecuted when a particular channel is extracted. Generally speaking, anysignal process is easier to form in a lower frequency region; thus,respectively overlapping the spread spectrum reference signal onto eachdata signal channel is more advantageous than overlapping the spreadspectrum reference signal, to be straddled onto all data channels.

In this embodiment, the frequency conversion is performed using twolocal-oscillation frequencies LO1 and LO2. Alternatively, the presentinvention can be applied to a frequency converter employing three ormore local-oscillation frequencies.

According to one aspect of the present invention, the phase-locked loopgenerates the subscriber station local-oscillation signal having the lowfrequency, of the plurality of local-oscillation signals, based on theintermediate frequency beacon signal, which is generated by mixing thepredetermined radio frequency beacon signal with the local-oscillationfrequency signal. Even if a frequency offset and a phase noise takeplace in the subscriber station local-oscillation signal having a highfrequency, of the plurality of local-oscillation signals, the frequencyoffset and the phase noise are compensated for or canceled out when thesignal in the first frequency band is mixed with the local-oscillationsignal having the low frequency. Specifically, even if the phase-lockedloop is used in the frequency converter to generate the lowlocal-oscillation frequency signal only, the frequency offset and thephase noise taking place in the remaining high frequencylocal-oscillation signals are compensated for or canceled out. A highfrequency accuracy thus results. This arrangement reduces the number ofbulky, costly and power-consuming phase-locked oscillators, typicallyused in the quasi millimeter band or the millimeter band. A simplified,compact frequency converter is thus provided, reducing both installationand operating costs. The overall frequency accuracy of the frequencyconverter, employing the phase-locked loop, is improved. Furthermore,since a frequency multiplier is dispensed with, the spuriouscharacteristics of the frequency converter are improved.

The phase-locked loop generates the local-oscillation signal having thelow frequency, of the plurality of local-oscillation signals so that theintermediate frequency beacon signal is synchronized with the signalhaving the predetermined frequency. Even when a frequency offset and aphase noise take place in a local-oscillation signal having a highfrequency, of the plurality of local-oscillation signals, the frequencyoffset and the phase noise are compensated for or canceled out when thesignal in the first frequency band is mixed with the local-oscillationsignal having the low frequency, of the plurality of local-oscillationsignals.

The phase-locked loop generates the local-oscillation signal having thelow frequency, of the plurality of local-oscillation signals, insynchronization with the beacon signal having the intermediatefrequency. Even when a frequency offset and a phase noise take place ina local-oscillation signal having a high frequency, of the plurality oflocal-oscillation signals, the frequency offset and the phase noise arecompensated for or canceled out when the signal in the first frequencyband is mixed with the local-oscillation signal having the lowfrequency, of the plurality of local-oscillation signals.

The construction of the receiver part of the frequency converter forconverting the radio frequency signal into the intermediate frequencysignal is simplified and is made compact. The installation cost andoperating cost of the converter are thus reduced.

The construction of the transmitter part of the frequency converter forconverting the intermediate frequency signal into the radio frequencysignal is simplified and is made compact. The installation cost andoperating cost of the converter are thus reduced.

According to another aspect of the present invention, the hub stationtransmits, to the subscriber station, the radio frequency beacon signalinto which the beacon signal having the hub station intermediatefrequency and the signal having the hub station local-oscillationfrequency are mixed. On the other hand, the subscriber stationcommunicates with the hub station by successively mixing the signal inthe first frequency band with the plurality of subscriber stationlocal-oscillation signals having the different subscriber stationlocal-oscillation frequencies. The phase-locked loop generates asubscriber station local-oscillation signal having a low frequency, ofthe plurality of subscriber station local-oscillation signals, based onthe intermediate frequency beacon signal, which is generated by mixingthe predetermined radio frequency beacon signal with thelocal-oscillation frequency signal. Therefore, even when a frequencyoffset and a phase noise take place in a local-oscillation signal havinga high frequency, of the plurality of local-oscillation signals, thefrequency offset and the phase noise are compensated for or canceled outwhen the signal in the first frequency band is mixed with thelocal-oscillation signal having the low frequency. Specifically, even ifthe phase-locked loop is used in the frequency converter to generate thelow local-oscillation frequency signal only, the frequency offset andthe phase noise taking place in the remaining high local-oscillationfrequency signals are compensated for or canceled out. A high frequencyaccuracy thus results. This arrangement reduces the number of bulky,costly and power-consuming phase-locked oscillators, typically used inthe quasi millimeter band or the millimeter band. A simplified, compactfrequency converter is thus provided, reducing both installation andoperating costs, and thereby lightening the burden on the subscriber.The overall frequency accuracy of the frequency converter, employing thephase-locked loop, is improved. Furthermore, since a frequencymultiplier is dispensed with, the spurious characteristics of thefrequency converter are improved.

In the subscriber station, the phase-locked loop generates thesubscriber station local-oscillation signal having the low frequency, ofthe plurality of subscriber station local-oscillation signals so thatthe beacon signal having the subscriber station intermediate frequencyis synchronized with the signal having a predetermined frequency. Evenwhen a frequency offset and a phase noise take place in a subscriberstation local-oscillation signal having a high frequency, of theplurality of subscriber station local-oscillation signals, and in thesignal in the hub station local-oscillation frequency, the frequencyoffset and the phase noise are compensated for or canceled out when thesignal in the first frequency band is mixed with the subscriber stationlocal-oscillation signal having the low frequency, of the plurality oflocal-oscillation signals.

The phase-locked loop generates the subscriber station local-oscillationsignal having the low frequency, of the plurality of subscriber stationlocal-oscillation signals, in synchronization with the beacon signalhaving the subscriber station intermediate frequency. Even when afrequency offset and a phase noise take place in a subscriber stationlocal-oscillation signal having a high frequency, of the plurality ofsubscriber station local-oscillation frequency signals, the frequencyoffset and the phase noise are compensated for or canceled out when thesignal in the first frequency band is mixed with the subscriber stationlocal-oscillation signal having the low frequency, of the plurality ofsubscriber station local-oscillation signals.

The construction of the receiver part of the subscriber station of theradio communications system for converting the radio frequency signalinto the intermediate frequency signal is simplified and is madecompact. The installation cost and operating cost of the radiocommunications system are reduced.

The construction of the transmitter part of the subscriber station ofthe radio communications system for converting the intermediatefrequency signal into the radio frequency signal is simplified and ismade compact. The installation cost and operating cost of the radiocommunications system are reduced.

Since the radio communications system employs the phase-locked loop forthe low frequency local-oscillation signal only, the overall frequencyaccuracy of the system is improved and the utilization of frequencies isimproved.

According to another aspect of the present invention, in the frequencyconverter, the frequency of the local-oscillation signal having the lowfrequency, of the plurality of local-oscillation signals, is set basedon the level of the reference signal that is obtained by despreading theintermediate frequency signal. When the reference signal is correctlydespread, the reference signal obtained through the despreading processexceeds a predetermined threshold level. The intermediate frequency thenis regarded as being coincident with the transmission frequency when thereference signal is spread. Even when a frequency offset and a phasenoise take place in a local-oscillation signal having a high frequency,of a plurality of local-oscillation frequency signals, the frequencyoffset and the phase noise are compensated for or canceled out, and ahigh accuracy level of frequency is achieved. Since the reference signalis overlapped in the same frequency band as that of data signal, throughthe spread spectrum modulation, with almost no effect incurred on thedata signal, no band outside the frequency band for the data signal isconsumed in a useless fashion, and generally limited frequency bands areefficiently utilized. Since there is no need for crystal oscillators orthe like, a simple and compact design may be promoted further, and theinstallation and operating costs of the converter are reduced.

The construction of the receiver part of the frequency converter forconverting the radio frequency signal into the intermediate frequencysignal is simplified and is made compact. The installation cost andoperating cost of the converter are reduced.

The construction of the transmitter part of the frequency converter forconverting the intermediate frequency signal into the radio frequencysignal is simplified and is made compact. The installation cost andoperating cost of the converter are reduced.

According to yet another aspect, in the radio communications system, thehub station transmits, to the subscriber station, the spread spectrumsignal that is obtained by subjecting the predetermined reference signalto the spread spectrum process, and by mixing the spread spectrumreference signal with the hub station local-oscillation frequencysignal. To communicate with the hub station, the subscriber stationconverts the signal in the first frequency band to the signal in thesecond frequency band, by successively mixing the signal in the firstfrequency band with the plurality of subscriber stationlocal-oscillation signals having the different frequencies. In thefrequency conversion process, the subscriber station local-oscillationsignal having the low frequency, of the plurality of subscriber stationlocal-oscillation signals, is set in frequency so that the level of thereference signal that is obtained by despreading the subscriber stationintermediate frequency signal exceeds a predetermined threshold. Whenthe reference signal is correctly despread, the reference signalobtained through the despreading process exceeds the predeterminedthreshold level. The subscriber station intermediate frequency then isregarded as being coincident with the transmission frequency when thereference signal is spread. Even when a frequency offset and a phasenoise take place in a subscriber station local-oscillation signal havinga high frequency, of a plurality of local-oscillation signals, thefrequency offset and the phase noise are compensated for or canceledout, and a high accuracy level of frequency is achieved. Since thereference signal is overlapped in the same frequency band as that ofdata signal, through the spread spectrum modulation, with almost noeffect incurred on the data signal, no band outside the frequency bandfor the data signal is consumed in a useless fashion, and generallylimited frequency bands are efficiently utilized. Since there is no needfor crystal oscillators or the like, a simple and compact design may bepromoted further, and the installation and operating costs of theconverter are reduced.

The construction of the receiver part of the subscriber station of theradio communications system for converting the radio frequency signalinto the intermediate frequency signal is simplified and is madecompact. The installation cost and operating cost of the radiocommunications system are reduced.

The construction of the transmitter part of the subscriber station ofthe radio communications system for converting the intermediatefrequency signal into the radio frequency signal is simplified and ismade compact. The installation cost and operating cost of the radiocommunications system are reduced.

Since the radio communications system employs the phase-locked loop forthe low local-oscillation frequency signal only, the overall frequencyaccuracy of the system is improved and the utilization of frequencies isimproved.

1. A radio frequency converter for converting a signal in a firstfrequency band to a signal in a second frequency band by successivelymixing the signal in the first frequency band with a plurality oflocal-oscillation signals having different frequencies, the frequencyconverter comprising an oscillator employing a dielectric resonator forproviding a first one of the local-oscillation signals in a quasimillimeter bandwidth or a millimeter bandwidth which contains afrequency drift; and a phase-locked loop, wherein the phase-locked loopgenerates at least a second one of the local-oscillation signals havinga lowest frequency among the plurality of local-oscillation signals,based on an intermediate frequency beacon signal, said intermediatefrequency beacon signal is generated by mixing a predetermined radiofrequency beacon signal generated from the signal in a first frequencyband with the first local-oscillation signals, and the intermediatefrequency beacon signal is mixed with the second one of thelocal-oscillation signals to compensate the frequency drift containedtherein, and wherein the first frequency band and the second frequencyband fall in a quasi millimeter bandwidth of 20–30 GHz or a millimeterbandwidth 30–300 GHz.
 2. A frequency converter according to claim 1,wherein the phase-locked loop generates the local-oscillation signalhaving the lowest frequency, of the plurality of local-oscillationsignals so that a difference between the frequency of the intermediatefrequency beacon signal and a predetermined reference frequency becomeszero.
 3. A frequency converter according to claim 1, wherein thephase-locked loop generates the local-oscillation signal having thelowest frequency, of the plurality of local-oscillation signals so thatthe intermediate frequency beacon signal is synchronized with a signalhaving a predetermined frequency.
 4. A frequency converter according toclaim 1, wherein the phase-locked loop synchronizes thelocal-oscillation signal having the lowest frequency, of the pluralityof local-oscillation signals, with the intermediate frequency beaconsignal.
 5. A frequency converter according to claim 1, wherein thephase-locked loop includes a reference crystal oscillator.
 6. A radiocommunications system comprising a hub station and at least onesubscriber station radio linked to the hub station, wherein thesubscriber station communicates with the hub station by converting asignal in a first frequency band to a signal in a second frequency bysuccessively mixing the signal in the first frequency band with aplurality of subscriber station local-oscillation signals havingdifferent frequencies, the hub station transmits, to the subscriberstation, a beacon signal having a radio frequency that is generated bymixing a signal having a hub station local-oscillation frequency with abeacon signal having a predetermined hub station intermediate frequency,and the subscriber station comprises an oscillator employing adielectric resonator for providing a first one of the subscriber stationlocal-oscillation signals in a quasi millimeter bandwidth or amillimeter bandwidth which contains a frequency drift; and aphase-locked loop, wherein the phase-locked loop generates at least asecond one of the subscriber station local-oscillation signals having alowest among the plurality of local-oscillation signals, based on anintermediate frequency beacon signal, said intermediate frequency beaconsignal generated by mixing the radio frequency beacon signal withanother one of the subscriber station local-oscillation signals, and theintermediate frequency beacon signal is mixed with the second one of thelocal-oscillation signals to compensate the frequency drift containedtherein, and wherein the first frequency band and the second frequencyband fall in a quasi millimeter bandwidth of 20–30 GHz or a millimeterbandwidth of 30–300 GHz.
 7. A radio communications system according toclaim 6, wherein the phase-locked loop generates the subscriber stationlocal-oscillation signal having the lowest frequency, of the pluralityof subscriber station local-oscillation signals so that a differencebetween the frequency of the subscriber station intermediate frequencybeacon signal and a predetermined reference frequency becomes zero.
 8. Aradio communications system according to claim 6, wherein thephase-locked loop generates the subscriber station local-oscillationsignal having the lowest frequency, of the plurality of subscriberstation local-oscillation signals so that the subscriber stationintermediate frequency beacon signal is synchronized with a signalhaving a predetermined frequency.
 9. A radio communications systemaccording to claim 6, wherein the phase-locked loop synchronizes thesubscriber station local-oscillation signal having the lowest frequency,of the plurality of subscriber station local-oscillation signals, withthe subscriber station intermediate frequency beacon signal.
 10. A radiocommunications system according to claim 6, wherein a predeterminedfrequency range used by the hub station and the subscriber station isdivided into a plurality of frequency channels.
 11. A radiocommunications system according to claim 6, wherein the phase-lockedloop includes a reference crystal oscillator.
 12. A radio frequencyconverter for converting a signal in a first frequency band which isabove a predetermined radio frequency to a signal in a second frequencyband which is above the predetermined radio frequency by successivelymixing the signal in the first frequency band with a plurality oflocal-oscillation signals having different frequencies, the frequencyconverter comprising an oscillator employing a dielectric resonator forproviding a first one of the local-oscillation signals in a quasimillimeter bandwidth or a millimeter bandwidth which contains afrequency drift; and a phase-locked loop, wherein a spread spectrumreference signal having a predetermined frequency and the firstlocal-oscillation frequency signal are mixed into an intermediatefrequency signal which is despread to result in a reference signal,wherein based on the reference signal, the phase-locked loop generatesat least a second one of the local-oscillation signals having a lowestfrequency among the plurality of local-oscillation signals having thedifferent frequencies is generated, and the intermediate frequencybeacon signal is mixed with the second one of the local-oscillationsignals to compensate the frequency drift contained therein, and whereinthe first frequency band and the second frequency band fall in a quasimillimeter bandwidth of 20–30 GHz or a millimeter bandwidth of 30–300GHz.
 13. A frequency converter according to claim 12, wherein thefrequency of the local-oscillation signal having the lowest frequency,of the plurality of local-oscillation signals, is determined based onthe level of the reference signal that is obtained by despreading theintermediate frequency signal.
 14. A frequency converter according toclaim 12, wherein the phase-locked loop includes a reference crystaloscillator.
 15. A radio communications system comprising a hub stationand at least one subscriber station radio linked to the hub station,wherein the subscriber station communicates with the hub station byconverting a signal in a first frequency band which is above apredetermined radio frequency to a signal in a second frequency bandwhich is above the predetermined radio frequency by successively mixingthe signal in the first frequency band with a plurality of subscriberstation local-oscillation signals having different frequencies, the hubstation transmits, to the subscriber station, a spread spectrum signalthat is generated by spreading a predetermined reference signal and thenby mixing a hub station local-oscillation frequency signal with thespread spectrum reference signal, and the subscriber station comprisesan oscillator employing a dielectric resonator for providing a first oneof the subscriber station local-oscillation signals in a quasimillimeter bandwidth or a millimeter bandwidth which contains afrequency drift; and a phase-locked loop, wherein the phase-locked loopgenerates at least a second one of the subscriber stationlocal-oscillation signal having a lowest frequency among the pluralityof subscriber station local-oscillation signals used in the subscriberstation from a reference signal that is obtained by despreading asubscriber station intermediate frequency signal which results frommixing the spread spectrum signal transmitted from the hub station andthe first subscriber station local-oscillation frequency signal, and theintermediate frequency beacon signal is mixed with the second one of thelocal-oscillation signals to compensate the frequency drift containedtherein, and wherein the first frequency band and the second frequencyband fall in a quasi millimeter bandwidth of 20–30 GHz or a millimeterbandwidth of 30–300 GHz.
 16. A radio communications system according toclaim 15, wherein the frequency of the subscriber stationlocal-oscillation signal having the lowest frequency, of the pluralityof subscriber station local-oscillation signals, is determined based onthe level of the reference signal that is obtained by despreading theintermediate frequency signal.
 17. A radio communications systemaccording to claim 15, wherein a predetermined frequency range used bythe hub station and the subscriber station is divided into a pluralityof frequency channels.
 18. A radio communications system according toclaim 15, wherein the phase-locked loop includes a reference crystaloscillator.